LTR-containing retrotransposons and retroviruses represent two related branches on the phylogenetic tree of reverse transcriptase (RT)-containing mobile genetic elements. Both types of elements use the RNA-dependent DNA polymerase, RT, to replicate their RNA genomes into double strand DNA which is then inserted into a host genome. The movement of retrotransposons within a genome can be harmful, resulting in genomic instability, cancer, sterility and mutations. On the other hand retrotransposons actively contribute to the evolution of their hosts' genomes. We have a long term interest in the replication mechanisms utilized by retrotransposon Tyl to maintain itself as an endogenous family of mobile elements in its host organism Saccharomyces cerevisiae. Tyl is a particularly useful experimental system to study transposition because it is so easily manipulated, because its natural host is such a genetically tractable organism, and because the steps in its replication are so closely related to the steps involved in retroviral replication. Since DNA polymerases replicate and transmit vital genetic information, the fidelity of their synthesis is a crucial biochemical determinant of their functional capacity. We have found that Tyl RT is an error prone polymerase, that generates errors at specific sites by a novel mechanism. We propose (Aim 1) to explore this new mutagenic mechanism genetically and biochemically, and determine the consequences of this process to Tyl replication fitness. Further, we will identify and characterize additional mutagenic mechanisms employed by Tyl RT during replication. We have created and are characterizing a novel mutation in the active site of Tyl RT which can still synthesize DNA but is blocked for transposition. This defect is suppressible by mutations in both the RNase H and polymerase domains of RT. We plan (Aim 2) to use this novel mutant to examine the distinctions between polymerization and replication, and more broadly, to understand the interactions between protein domains during Tyl replication. Further we will determine whether the properties of this novel mutant are generalizable to the ubiquitous family of retrotransposons whose RTs have not previously been studied.